Under normal physiological conditions, plasma calcium and phosphate are present at concentrations close to supersaturation levels and, as a result, may be expected to precipitate in soft tissue (e.g., blood vessels) as crystalline hydroxyapatite. The observation that this process does not occur in healthy subjects suggested the presence of potent chemical and biologic means for blocking pathologic calcification (Price, et al. (2002) “Discovery of a High Molecular Weight Complex of Calcium, Phosphate, Fetuin, and Matrix-Carboxyglutamic Acid Protein in the Serum of Etidronate-treated Rats,” JOURNAL BIOL. CHEM. 277 (6): 3926-3934).
When the suppression of calcification is disrupted, such as in subjects with diabetes and chronic kidney disease (CKD), pathologic calcification of soft tissue (e.g., blood vessels) can occur. It is understood that diabetes can lead to CKD and end stage renal disease (ESRD), which is characterized by uremia. Uremia can promote the oxidation of Vitamin K hydroquinone (KH2), thereby disrupting the cyclic regeneration of Vitamin K, among other effects. (See, FIG. 1.) In addition, certain treatments can cause or contribute to Vitamin K dysregulation, including warfarin-based anticoagulant therapy and statin therapy. The loss of functional Vitamin K results in the loss of important regulators of mineralization, leading to pathologic calcification of tissue. In the case of arterial calcification, intradermal microvascular thrombosis is observed to occur, resulting in small vessel blockages and surrounding tissue death.
Vitamin K is an essential enzymatic co-factor that is required for post-translational modifications of Vitamin K-dependent (VKD) proteins. A number of VKD proteins are clinically relevant to CKD and ESRD patients, and include, for example, central coagulation factors such as factors II, VII, IX, and X and intercellular matrix proteins such as Matrix Gla Protein (MGP) activated protein C and osteocalcin. Vitamin K is a group of fat soluble vitamins, which include, among other things, Vitamin K1 (also known as phylloquinone), which is made by plants, and Vitamin K2 (also known as menaquinone), which is made by bacteria in gut flora. It is understood that the isoprenoid chain in Vitamin K2 can contain from 4 to 12 repeating isoprenoid units. For example, menaquinone-4 (or MK-4) contains four isoprenoid units whereas menaquinone-7 (or MK-7) contains seven isoprenoid units.
With regard to menaquinone-7 (MK-7), under normal conditions MK-7 is reduced to menaquinol-7 (MKH2-7) (a form of Vitamin K hydroquinone) by an NADPH-dependent reductase enzyme or enzymes (e.g., quinone oxidoreductase). Only the reduced form of MK-7 (namely MKH2-7) functions as a co-factor for the enzyme gamma glutamate carboxylase (GGCX), which catalyzes the carboxylation of Vitamin K-dependent proteins. (See, FIGS. 1 and 2.) The enzymatic carboxylation of glutamate residues results in oxidation of MKH2-7 to a 2,3-epoxide form (MK-7 2,3-epoxide). The final step of the Vitamin K cycle requires the enzymatic reduction of Vitamin MK-7 2,3-epoxide back to MK-7 by Vitamin K epoxide reductase complex subunit 1 (VKORC1, also referred to as VKOR. In some tissues, the paralog VKORC1L1 (VKORC1-Like-1) may also perform this catalytic reaction. It is believed that warfarin blocks both the generation of MKH2-7, the active form of Vitamin K2, as well as the regeneration of MK-7 from Vitamin MK-7 2,3-epoxide, which may lead to the higher incidence of calcification seen among patients receiving warfarin therapy.
Despite efforts to date, there is a need for new clinical approaches to prevent and/or reverse pathologic calcification. In particular, there is a need for new clinical approaches to prevent and/or reverse pathologic calcification in subjects with diabetes, CKD, ESRD, and subjects receiving anticoagulant and/or statin therapy.